Hereinafter, the term “front” refers to a spark discharge side with respect to the direction of an axis of a spark plug; and the term “rear” refers to a side opposite the front side.
A spark plug is mounted to an internal combustion engine (sometimes simply referred to as “engine”) etc. and used to ignite an air-fuel mixture in a combustion chamber of the engine. The spark plug generally includes an insulator formed with an axial hole in the direction of an axis of the spark plug, a center electrode inserted in a front side of the axial hole, a metal shell arranged on an outer circumference of the insulator and a ground electrode joined to a front end portion of the metal shell. The ground electrode is bent at a substantially middle portion thereof toward the center electrode such that a distal end portion of the ground electrode faces a front end portion of the center electrode to define a spark discharge gap between the distal end portion of the ground electrode and the front end portion of the center electrode.
For improvements in the corrosion resistance of the metal shell and the ground electrode, there is a case where a thin coating layer of nickel-based metal material (hereinafter simply referred to as “nickel layer”) is applied to surfaces of the metal shell and the ground electrode. It is common practice to join the ground electrode to the metal shell, and then, apply the nickel layer to the resulting joined unit by a plating process. In this case, the nickel layer is formed on the entire surfaces of the metal shell and the ground electrode.
The nickel layer may be separated when the ground electrode coated with the nickel layer is subjected to bending. The occurrence of such separation of the nickel layer becomes a cause of abnormal spark discharge, called “lateral spark”, between the separated part of the nickel layer and the center electrode and leads to deterioration in the ignition performance of the spark plug.
Further, there is recently a case where a tip of highly wear-resistant metal material is bonded to the distal end portion of the ground electrode for improvements in the wear resistance of the ground electrode and in the ignition performance of the spark plug. However, the bondability of the tip to the ground electrode may not be sufficient when the tip bonding site of the ground electrode is being coated with the nickel layer. It is thus conceivable to remove the nickel layer from at least the distal end portion of the ground electrode. For example, Japanese Laid-Open Patent Publication No. 2003-123937 discloses a technique of removing the nickel layer by grinding using a given grinding jig. Japanese Laid-Open Patent Publication No. 2012-15126 teaches a technique of applying a masking layer to at least the distal end portion of the ground electrode before the plating process and, after the plating process, removing the nickel layer along with the masking layer.